In an adsorption heat pump, the adsorbent having an adsorbate, e.g., water, adsorbed thereon is heated to desorb the adsorbate in order to regenerate the adsorbent, and the adsorbent dried is cooled to a temperature to be used for adsorbate adsorption before being used for adsorbate adsorption again.
Absorption type heat pumps in which waste heat or heat having a relatively high temperature (120° C. or higher) is utilized as a heat source for adsorbent regeneration have already come into practical use. However, since the heat obtained from cogeneration apparatus, fuel cells, cooling water for automotive engines, solar energy, or the like generally has a relatively low temperature of 100° C. or below, it cannot be utilized as a heat source for driving the absorption type heat pumps presently in practical use. It has been desired to effectively utilize low-temperature waste heat of 100° C. or lower, especially from 60° C. to 80° C. In particular, there is a strong desire for the practical use thereof in motor vehicles which generate waste heat in large quantities.
In adsorption heat pumps, the adsorption properties required of adsorbents vary considerably depending on the temperatures of utilizable heat sources even though the heat pumps operate on the same principle. For example, the temperatures of higher-temperature-side heat sources are from 60° C. to 80° C. in the case of waste heat from gas engine cogeneration and from solid polymer type fuel cells and are from 85° C. to 90° C. in the case of cooling water for automotive engines. The temperatures of cooling-side heat sources also vary depending on the places where the apparatus is installed. For example, the cooling-side heat source temperatures in the case of motor vehicles are temperatures obtained with the radiators, while those in buildings, houses, and the like are the temperatures of water-cooling towers, river water, etc. Namely, the operating temperatures for an adsorption heat pump are as follows. In the case of installation in buildings or the like, the lower-side temperatures are from 25° C. to 35° C. and the higher-side temperatures are from 60° C. to 80° C. In the case of installation in motor vehicles or the like, the lower-side temperatures are about from 30° C. to 45° C. and the higher-side temperatures are about from 85° C. to 90° C. There is hence a desire for an apparatus capable of being operated even with a small temperature difference between the lower-temperature-side heat source and the higher-temperature-side heat source so as to effectively utilize waste heat. An adsorbent to be applied to such apparatus is also desired.
Typical adsorbents known as adsorbents for adsorption heat pumps are zeolite 13X and A-form silica gel.
Recently, zeolites are being investigated, such as a mesoporous molecular sieve (e.g., FSM-10) synthesized using the micellar structure of a surfactant as a template (Japanese Patent Laid-Open No. 178292/1997) and a porous aluminum phosphate molecular sieve for use as a desiccant material commonly referred to as AlPO4 (Japanese Patent Laid-Open No. 197439/1999).
It has already been reported that the temperature dependence of adsorption properties is important for the adsorbents for adsorption heat pumps (Ragaku Kôgaku Ronbun-shû, Vol. 19, No. 6 (1993), pp. 1165-1170). There is a report therein that SG3 (manufactured by Fuji Silysia Ltd.) shows a large temperature dependence and SG1 (manufactured by the same) does not.
Furthermore, it has been reported that the adsorption performance of AlPO4-5, which is a porous aluminum phosphate molecular sieve, depends on temperature. Specifically, the adsorption performance at 25° C. and that at 30° C. are shown (Colloid Polym Sci, 277 (1999) pp. 83-88). Likewise, the temperature dependence of AlPO4-5 has been reported; adsorption isotherms obtained in an adsorption process at 20° C., 25° C., 30° C., 35° C., and 40° C. are shown (Dai-16-kai Zeoraito Kenkyû Happyô Kai Kôen Yokô-shû, p. 91; Nov. 21 and 22, 2000).
Use of various adsorbents in adsorption heat pumps is being investigated. However, our investigations revealed that there is yet room for improvement in adsorption performance so as to enable application to an apparatus capable of being operated even with a small temperature difference between the lower-temperature-side heat source and the higher-temperature-side heat source.